1. Field of the Invention
The present invention relates to a seat load detection apparatus.
2. Description of the Related Art
A known seat load detection apparatus is described in, for example, Japanese Unexamined Patent Application Publication No. 2003-83798. This seat load detection apparatus includes cantilever type rectangular strain elements 21 each including a strain gauge 22. More specifically, one end of each of the strain elements 21 is fixed to the seating portion of the seat with a bracket 23 having an L-shaped cross-section, whereas the other end is fixed to the vehicle body side of the seat with another bracket 24 having an L-shaped cross-section. When one of the ends of the strain element 21 is bent with respect to the other end due to load applied on the seat, the output signal from the strain gauge 22 changes in accordance with the amount of bending, enabling the seat load detection apparatus to detect the applied load.
However, for such a known seat load detection apparatus, brackets 23 and 24 having L-shaped cross-sections are required for mounting the apparatus on a seat, increasing the overall size and weight of the apparatus. Furthermore, since the strain elements 21 have a cantilever structure, stoppers 25a and 27a are required to limit the amount of distortion in the vertical direction of the strain elements 21 in order to maintain sufficient strength to support the seat. Therefore, the known seat load detection apparatus has a complex structure.
Such a known seat load detection apparatus having a reduced size is also described in Japanese Unexamined Patent Application Publication No. 2004-268620. This known seat load detection apparatus includes a bolt 30, which forms a single unit with a flange 20 in contact with and mounted on an upper rail 42, and a disc-shaped strain element 14, which is attached to a depression 24 formed on the flange 20. The bolt 30 is provided orthogonally to the flange 20. The bolt 30 is passed through a nut 66 and attached to a bracket 48, which is used to install the seat. A sensor 12 for detecting the load applied to the bolt 30 in the axial direction is disposed on the strain element 14. According to this structure, when the inner circumference of the strain element 14 bends with respect to the outer circumference due to the load applied in the axial direction of the bolt 30 on the seat, the load is detected as a change in the output signal from the sensor 12 in accordance with the degree of bending. Although the strain element 14 has a small size, the diaphragm structure of the strain element 14 provides sufficient strength for supporting the seat.
The seat load detection apparatus according to Japanese Unexamined Patent Application Publication No. 2004-268620 requires an electric circuit for processing the output signal from the sensor to detect the load applied on the seat. Such an electric circuit is mounted on a circuit board provided separately from the seat load detection apparatus and is electrically connected to the sensor via leads. Therefore, to dispose the circuit board including the electric circuit in the vicinity of the sensor 12, space for preventing the circuit board from interfering with the flange 20 must be provided. As a result, the overall size of the seat load detection apparatus becomes large. If, instead, the circuit board including the electric circuit is disposed separately from the sensor in order to prevent interference with the flange 20, the electrical wiring becomes complicated.
The assignee has considered providing a bolt unit forming a unit including a bolt and a substrate (circuit board) on which an electric circuit can be mounted so as to minimize the space required for proving the substrate. However, in such a case, the bolt protrudes from the flat substrate. Therefore it is difficult to mount the electric circuit on the substrate unless the seat load detection apparatus has a structure that is large enough to include the three-dimensional bolt unit.